Composite colour television systems, such as NTSC and PAL are designed so that the luminance and chrominance components of the signal can occupy the same spectrum and yet be separated with acceptable levels of crosstalk. Use of the composite signal for display always requires it to be separated (decoded) into primary colour components (red, green and blue); many other processing operations require at least the separation of the chrominance from the luminance, and often, further decoding of the chrominance.
The choice of decoding method for a particular application depends on the acceptable level of crosstalk between chrominance and luminance, and the degree of distortion to the wanted chrominance and luminance as a result of the separation process. Where images are to be displayed on large screens, or must stand comparison with images derived from film, the very highest level of chrominance and luminance separation is necessary, with negligible crosstalk and distortion.
High quality composite decoders exploit the fact that the chrominance is modulated onto a high-spatial-frequency, moving diagonal carrier, which is unlikely to be replicated by real luminance. By using contributions from several lines and fields it is possible to create comb-filters which allocate different parts of the signal spectrum to luminance and chrominance and so achieve separation with little crosstalk on typical pictures. However, it is frequently necessary to “adapt” the frequency characteristics of the filters in dependence upon the type of picture material, so as to avoid crosstalk or distortion of critical material.
European patent EP1175102 presents a novel method of doing this by using a fast Fourier transform (FFT) to convert the signal into the frequency domain and then investigating the symmetry of the spectrum with respect to subcarrier frequency. Because the chrominance components are amplitude modulated, each Fourier component of the baseband chrominance results in a pair of equal-amplitude sidebands symmetrically disposed about the subcarrier frequency. By comparing each frequency component with the corresponding component which would correspond to the opposite chrominance sideband, it is possible to determine whether the particular component represents luminance or chrominance.
The cited patent does not explain how to apply the technique to NTSC, and is restricted to applications where the signal is sampled at a multiple of the subcarrier frequency.